The present invention relates generally to virtual reality technology, and more specifically to improved optical systems for virtual reality head mounted displays.
Virtual reality has become the focus of increased attention in recent years. Virtual reality systems are already in practical use in various fields, being utilized by, for example, engineers and architects for modelling and testing new designs, by doctors to practice an operation before it is conducted, and by military experts to simulate battlefield conditions. Such virtual reality systems employ sophisticated computers utilizing multisensory inputs and outputs to create an interactive virtual world in which a product, procedure or human response may be tested without the risks inherent in real-world trials.
In order to accurately simulate human interaction with a virtual environment, be it the inside of an internal combustion engine, an operating room or a battlefield, virtual reality systems aim to facilitate input and output of information representing human senses. In many situations, among all the human senses, sight is perhaps most useful as an evaluative tool. Accordingly, an optical system for visualization is an important part of most virtual reality systems.
Visualization in virtual reality systems is typically accomplished by means of a head-mounted display (HMD) mounted in a helmet or goggles worn on the user's head with an electronic display mounted in front of the user's eyes. A housing surrounds the display to reduce or eliminate ambient light. In some systems, stereoscopic vision is facilitated through the use of a pair of displays, each aligned with one of the user's eyes. More compact configurations are achieved by mounting the displays relatively close to the eyes, typically within 3 inches, and providing short focal length, wide-angle lenses between each eye and the associated display.
Larger displays may be used at relatively close range through the use of Fresnel lenses to provide wider field of view. Fresnel lenses have their curves reduced from a single large curved face to a series of equivalently curved rings or steps. If an ordinary, continuously curved lens is compared to a Fresnel lens, the angle of the surface of the lens face at any point relative to the XY plane through the lens would be the same in both lens types for an equivalent focal length lens; however, the Fresnel lens would appear flat with ridges forming rings.
In addition to the loss in weight produced by this loss in volume (while retaining the same focal length), the Fresnel can be cast in plastic rather than glass, further reducing the weight.
Since the Fresnel condenser pair is comprised of 2 flat (ridged) objects, instead of 2 objects with extreme curvature, they can be placed in closer proximity without contacting one another, thus reducing front projection.
Vision is further improved by the use of multiple lenses. A pair of lenses may be arranged in parallel in a configuration called a short focal length condenser pair. This arrangement is limited in how thin it can be made by the curvature of the lenses. Once the lenses touch each other, the total thickness cannot be reduced further, putting a lower limit on the space occupied by the lens pair.
In a head mounted display, the design goals include light weight and limited front projection (the distance from the face of the wearer to the outer extent of the housing). This front projection is large when using conventional lenses due to the thickness and curvature of the lenses, even though the focal length (the distance between the eye and the display) of the system, perceived to be 60 inches or so, is actually only about 11/2 inches.
While virtual reality visualization systems have shown promise, known systems have suffered from certain drawbacks. Primary among these has been the poor quality of the image produced by the optics in known systems. Part of this problem stems from the limited field of view afforded by known optical systems. In such systems, the displays and lenses have generally been mounted in a plane perpendicular to the optical axis, limiting the degree of wide-angle visualization possible.
In addition, efforts to provide wider field of view through the use of two parallel Fresnel lenses have been compromised by the effects of Moire interference, creating image-degrading diffraction patterns. Standard Fresnels have ridges as described above which define the lens curvature spaced at equal distances, in other words, steps of equal size (but different curvature angle .theta.). When two identical Fresnels are stacked in a condenser pair, the wave fronts are slightly distorted by each lens. In this configuration the "steps" result in light passing through the thicker cross sections more slowly than through the thinner areas (because light moves slower in solid space than in air). These wave front distortions, which are identical because the lens ridges of each are identical, each add and a subtract from the relative power of the other, resulting in a pattern of strengthened and weakened power from point to point. This is known as Moire interference, and is perceived as a series of light and dark tings, greatly accentuated if the lens axes are de centered from each other. This principle can be demonstrated by moving two pieces of window screen past each other.
Image quality in known systems is further degraded by the inability to adjust the interoptic distance between the left and right lenses to compensate for the different interpupillary distances of various users. Moreover, while some known virtual reality HMDs have allowed adjustment of the roll (vertical perpendicularity) of the lenses and displays relative to the user's eyes, such systems have suffered from the inability to adjust the height of the displays and lenses relative to the eyes, along with the roll.
For these reasons, an improved optical system for a virtual reality head mounted display is desired which will overcome the disadvantages of previous systems. In particular, the optical system should provide wider field of view for improved image quality at the outer edges of the displays, as well as permitting the use of larger displays. The optical system should have improved image quality by reducing the interference effects of parallel lenses. The optical system should further allow adjustment of the interoptic distance between the left and right lenses, as well as the height and roll of the displays and lenses relative to the eyes, to optimize image quality for each user.